PHYLUM ARTHROPOD A 



213 



which, like the body itself, are enclosed in a hard, jointed 

 exoskeleton, their movable segments being termed 

 podomeres. 



The body of the Crayfish (Fig. 114) is divided into two 

 regions an anterior, the cephalothorax (cth.\ which is un- 

 jointed, and is covered by a broad shield or carapace and 

 a posterior, the abdomen (ah.}, which is divided into distinct 

 segments, movable upon one another in a vertical plane. 



FIG. 114. Astacus fluviatilis, side view of male. '. antennule ; a 2 , antenna; 

 ab. abdomen ; cth. cephalothorax : kd. gill-cover ; r. rostrum ; 8, third maxilli - 

 pede ; 9, first leg ; 10 13, remaining legs ; /<?, uropod ; XIV, first abdominal 

 segment; XIX, sixth abdominal segment. (From Lang's Comparative 

 Anatomy r , after Huxley.) 



The cephalothorax is again divided into two regions an 

 anterior, the head, and a posterior, the thorax by a trans- 

 verse depression, the cervical groove. The carapace is 

 developed from the dorsal and lateral regions of both head 

 and thorax, and is free only at the sides of the thorax, where 

 it forms a flap or gill-cover (kd.) on each side, separated 

 from the actual body- wall by a narrow space in which 

 the gills are contained. The carapace is strongly im- 



